There is a global demand to reduce energy consumption of image forming apparatuses. Some conventional image forming apparatuses have been designed to reuse the heat that is generated during the image formation.
Specifically, Japanese Patent Application Laid-Open No. H05-19654 and Japanese Patent Application Laid-Open No. 2000-338803 disclose image forming apparatuses that use a heat pipe to reuse the heat generated during the image formation for image fixation. These image forming apparatuses include a heat sink, which is positioned downstream of the section where the recording medium is heated to a melting point of toner to fuse toner images onto the recording medium. Moreover, a radiator is positioned upstream of the section where the heat fusion is performed.
These apparatuses have been designed so that the heat sink of the heat pipe absorbs the heat from the recording medium that has passed through the heat fusion section, and the radiator of the heat pipe releases the absorbed heat onto another recording medium as the another recording medium passes through the heat fusion section. This design allows thermal energy obtained from both the toner image fixing operation and the post-fixed recording medium to be transferred onto the recording medium prior to fixing of toner images. By absorbing the heat from the toner fixated images and the recording medium after passing through the heat fusion section, incidence of an offset (smearing) on the recording medium can be reduced if not prevented. On the other hand, preliminary heating of the unfixed toner images and the recording medium by the radiator before reaching the heat fusion section reduces heat energy required for fusing the toner images in the heat fusion section. Thus, the reuse of heat for the image fixing process for preliminary heating reduces the energy consumption.
However, the heat pipe in the conventional technology was originally designed to ensure efficient heat transfer from the heat sink to the radiator using elements that have extremely high thermal-conductivity. Moreover, heat transfer by thermal conduction causes heat to be transferred from a higher-temperature location toward a lower-temperature location, but not from the lower-temperature location to the higher-temperature location. Accordingly, when the heat pipe is used, the temperature of the heat sink can not rise higher than the temperature of the radiator. Moreover, the temperature of the heat fusion section increases to its upper limit during the toner fixation process, reaching a higher temperature than that of the heat sink of the heat pipe. As such, the radiator cannot be positioned in the heat fusion section, because the heat fusion section has a temperature that is higher than that of the heat sink. Therefore, the reuse of waste heat by the heat pipe for the toner fixing operation can be applied only to the preheating operation, in which the temperature of the recording medium with unfixed toner images is raised to a particular level; but not to the heating operation that occurs in the heat fusion section.
The typically close proximity of the preheating unit (radiator) and the heat fusion section causes heat to be transferred from the heat fusion section to the radiator by air conduction, raising the temperature of the radiator. As the temperature of the radiator rises and a temperature differential between the radiator and the heat sink becomes smaller, heat cannot be transferred from the heat sink to the radiator. On the other hand, if the radiator is positioned away from the heat fusion section, to avoid convection heating by the heat fusion section, then there is less efficient reuse of heat since the recording medium releases heat between the radiator and the heat fusion section.
Japanese Patent Application Laid-Open Publication No. 2001-42672 discloses an image forming apparatus that use Peltier elements to cool the surfaces of a conveying belt and a recording medium on a conveying belt downstream of the heat fusion region. In this conventional image forming apparatus, the heating member is positioned upstream from the heat fusion region in order to come into contact with the conveying belt. It is possible for this image forming apparatus to preheat the conveying belt by heat from the heating member. However, this heating member uses fans, heat sinks, and the like which release large amounts of heat into the air; and is an inefficient use of heat even when used as an auxiliary heating unit.